The present invention relates generally to methods of power control for wireless communication systems and, more particularly, to methods of forward link power control during soft handoff.
Numerous access schemes exist to allow multiple users to share a communication medium. One such access scheme is known as Code Division Multiple Access (CDMA). In CDMA systems, multiple users share the same carrier frequency and may transmit simultaneously. Each user has its own pseudo-noise (PN) sequence, which is approximately orthogonal to the PN sequences of other users. Transmissions to or from individual users are imprinted with that user's PN sequence. The receiver selects the desired signal, which combines with unwanted signal during transmission, by correlating the received signal with the PN sequence of the desired signal. All other undesired signals are spread by the PN sequence and appear as noise to the receiver.
Two standards for CDMA systems currently used in the United States are the IS-95 standard and the cdma2000 standard, which are described in specifications published by the Telecommunications Industry Association and Electronics Industry Association (TIA/EIA). The IS-95 standard was developed primarily for voice services and low rate data services. cdma2000 is a spread-spectrum radio interface that uses CDMA technology to satisfy the needs of third generation wireless communication systems. A significant benefit of cdma2000 is the ability to provide high-rate data services, in addition to voice services, for web browsing and other high data rate applications. In web browsing, the amount of traffic on the forward link is typically much larger than amount of traffic on the reverse link. Therefore, it is particularly important to increase spectrum efficiency, particularly on the forward link.
CDMA systems are inherently interference-limited systems. Since all mobile stations operate at the same frequency, internal interference generated within the system plays a critical role in determining system capacity and voice quality. Two closely related techniques used in CDMA systems to reduce interference and thereby increase system capacity are power control and soft handoffs.
Power control is used on the forward link in CDMA systems to control the power of signals received at each mobile station. To maximize system capacity, the system allocates power to individual mobile stations according to the interference levels at each mobile station. The purpose of forward link power control is to ensure that each mobile station receives signals on the forward link at a desired minimum signal to interference ratio (SIR). The minimum SIR is chosen by the system operator to ensure a desired quality of service. Thus, the base stations transmit with highest power to those mobile stations with the highest interference levels, and with lesser power to mobile stations with lower levels of interference. In CDMA systems, the system capacity is maximized if the transmit power level of each base station is controlled so that its signals arrive at the mobile station receiver with the minimum required SIR.
As the mobile station moves around within the network, the channel conditions change continuously due to fast and slow fading, shadowing, number of users, external interference, and other factors. Power control algorithms dynamically control the transmitted power on the forward link to maintain the minimum SIR at the mobile station under all conditions. Two forward link power control mechanisms are employed in conventional CDMA systems: slow forward power control and fast forward power control.
In slow forward power control the mobile station keeps track of the number of error frames and if the number of error frames over the designated period exceeds a threshold, the mobile station sends a Power Measurement Report Message (PMRM) to the base station containing the total number of frames received in the period, the number of error frames, and the frame error rate (FER). On receiving the PMRM, the base station adjusts its transmit power based on the reported frame error rate.
Fading sources in mobile radio systems require much faster power control than is possible with slow forward power control. Therefore, it is common to also provide a mechanism for fast forward power control. In fast forward power control, the mobile station compares the SIR of the received signals to a reference SIR and signals the base station to either increase or decrease its transmit power. The power control commands typically comprise power control bits (PCBs), which are sent at a rate of 800 bps. A bit value of “1” signals the base station to decrease its transmit power. A bit value of “0” signals the base station to increase its transmit power. Upon receipt of a power control command from the mobile station, the base station steps its transmit power up or down by a fixed amount, referred to as the step size.
Another technique used in CDMA communication systems to reduce interference is known as a soft handoff. A handoff is the act of transferring support for a mobile station from one base station to another when the mobile station moves between cells. In a traditional “hard” handoff, the connection to the current base station is broken and a connection is made with the new base station to resume communication with the mobile station. This is known as a “break before make” handoff. Because all base stations in a CDMA system use the same frequency, it is possible with the connection to the new base station before terminating the connection with the current base station. This is known as a “make before break” or “soft” handoff. A soft handoff requires less power, which reduces interference and increases system capacity. The set of base stations serving a given mobile station during a soft handoff is referred to as the active set for that mobile station.
During a soft handoff, each base station in the active set for a mobile station transmits the same data to the mobile station. Ideally, each base station in the active set transmits signals to the mobile station at the same transmit power level. System interference is minimized when the transmit power level of all base stations in the active set for a mobile terminal is the same, i.e. balanced. The mobile station combines the received signals from all of the base stations in a diversity combiner. The mobile station determines the power control commands to send by comparing the SIR of the combined signals to the reference SIR. Since the power control commands are sent to all of the base stations in the active set, the balance of the transmit power is maintained between base stations in ideal conditions.
Unfortunately, ideal conditions are not the norm in wireless communication systems and the power control commands transmitted by the mobile stations are often subject to reception errors. When reception errors occur, the base stations may not control transmit power correctly, and the balance of transmit power may not be maintained between the base stations. Imbalance may occur, for example, if one of the base stations fails to correctly receive the power control command from the mobile station while the other base stations adjust their transmit power. This imbalance, if not cancelled by additional reception errors, increases interference and results in loss of traffic capacity in the downlink.